She might dislike you.

Or she might think you like someone else. If you've recently been dating someone else, and word hasn't exactly gotten around that you two aren't dating any more, she may think you're not available. Or if you have a girl who happens to be a close friend, but you're not dating, maybe she doesn't know that. There's nothing worse than professing your undying like for someone, either directly, "Hey, I like you. Want to go out?" or indirectly, "A bunch of us are going to a movie tomorrow. Want to come along?" only to find out that he or she isn't emotionally available. It's about the most embarrassing thing that can happen, and she's smart to try to keep it from happening to her!

She could also be shy. Girls are great at talking to other girls, and pretty good at talking to guys they're not interested in. But get a girl near a guy she likes, and she is likely to completely, utterly forget what her native language is! Suddenly, she can't think of anything to say that a) isnt' stupid or b) won't give a clue that she's in serious crush-mode. God forbid the object of her crush find out! Because what if he doesn't feel the same way?

Also, she might not feel comfortable coming up to you and talking to you, especially if she likes you and feels tongue-tied, if she only sees you when you're surrounded by your friends. Try to arrange some alone-time when and where you know she'll be, and see if that changes things for you.

Have you tried talking to her? Maybe she's waiting for you to break the ice. It doesn't have to be a particularly long or serious conversation. If you two share a class, ask her how she did on the last test. If there's something big or really newsworthy going on in your community, ask her how she feels about it. If you know she has a pet, or a particular hobby, get her to talk about it. Girls, like everyone else, like to talk about themselves, and getting her to do so will show her what a great listener you are. Girls like great listeners.

She might not know you exist. You might not be on her radar. I'm sure there were a lot of guys I didn't talk to in high school because I didn't know them, or at least didn't know them well enough to walk up and start a conversation! Say "hi" whenever you see her. If she's involved in some sort of activity, try to get involved too, whether it's a club, yearbook staff, or whatever. Get on her radar, and she just might open up!

If nothing else works, ask her. Say, "Do I bother you? Do you hate my guts or something?" Arrange an exit strategy with a good friend so if she says "yes" you're not stuck standing there like an idiot.

Good luck, and let me know how it comes out!